When microwave signals are relayed from transmitters to receivers, the respective transmitting and receiving antennae are oriented for either horizontal or vertical polarization of the signal wave. In various types of equipment, such as used with satellites for relaying microwave signals for television, the equipment may utilize both horizontal and vertical polarization, both on ground stations and on the satellite transponders. This has the beneficial effect of doubling the available channels for a fixed number of frequency bands. Typically, odd numbered transponders (1, 3, 5, etc.) utilize vertical polarization while the even numbered transponders (2, 4, 6, etc.) utilize horizontal polarization. This method of polarization change between adjacent transponders acts to produce increased discrimination and reduced interference that might cause deterioration of the signal from the desired transponder. However, at a ground based receiver or "earth station" proper reception of the desired signals requires that the polarization of the antenna correspond to that of the transponder from which the signals are being sent back to earth. Thus, if the desired signal is vertically polarized, then the earth station antenna must likewise be vertically polarized. Conversely, if the desired signal from the transponder is horizontally polarized, the earth station antenna must also be horizontally polarized.
One approach to providing for reception of both horizontally and vertically polarized signals has been the utilization of dual receiver waveguides, one being horizontally polarized and the other being vertically polarized. Such dual polarization of antennas, however, requires duplication of expensive components, which may not be practical for certain price-sensitive markets, such as the home satellite television antenna market.
Efforts at providing simplified dual polarization receiving antennae have been made in the past. These are generally directed to the provision of a septum extending transversely across a circular waveguide, with the outer or signal receiving end being oriented normal to the direction of polarization of an incoming microwave signal. In this orientation the septum will not block or materially attenuate the incoming microwave nor cause reflections to occur as long as the septum is a relatively thin element. After the microwave has passed by the septum it will reform into a wave identical to that of the original, with the electric field lines being at all points normal to the septum. If the outer end of the septum is twisted, with the signal receiving entrance portion being positioned normal to the polarization of the incoming wave, the polarization of the wave passing through the waveguide can be rotated by virtue of the electric field lines remaining normal to the septum. Thus, by so twisting the septum the polarization of the signal as it passes through the waveguide of the receiving antenna can effectively be rotated to a relationship convenient for the remainder of the waveguide system.
Typical of prior art devices for rotating polarization of microwave have been structures shown in the patent to Raiman, U.S. Pat. No. 4,503,379 and to Bleackley, U.S. Pat. No. 3,296,558. Raiman teaches a septum formed of a continuous, serpentine-shaped, electrically conductive filament with the outer, signal receiving end being rotatable relative to a fixed inner portion and using a remotely controlled motor to rotate that outer portion. Bleackley teaches a structure having a plurality of pins mounted to a central torsion wire with the inner end of the wire being fixed and the outer end of the wire being rotatable about the axis of the torsion wire by rotation of a permanent magnet about the outside of the waveguide.
While these and other prior art structures have provided some capability for rotating the polarization of signals, they have both suffered from the complexity and expense that is generally attendant upon the manner in which they are fabricated and in which the elements are rotated. These expenses and inconveniences have limited their usefulness in price-sensitive applications.